Electromagnetic pickups are devices that create a magnetic field in which strings of a musical instrument such as an electric guitar vibrate thereby disturbing the magnetic flux lines of the magnetic field. The pickups have at least one coil of wire which is connected to an amplifier. The disturbed, i.e., moving, flux lines caused by the vibrating strings cause minute electrical currents to flow in the wires of the coil, and these currents, cause a tiny voltage varying signal at the input to the power amplifier to which the coil is connected which reproduces the vibration of the strings electrically. This voltage is amplified to create a signal which drives speakers which reproduce the sounds made by the strings but at a much higher volume.
This would be all there is to it except for the problem of electrical noise. Electrical motors, 60 cycle per second utility system power and harmonics thereof, car ignitions and many other things cause electromagnetic flux variations in the atmosphere practically everywhere. This is in fact the basic theory of how radio waves propagate. These electromagnetic flux variations caused by things other than string vibration in the magnetic field of the pickup also cause electrical currents to flow in the pickup's coil. These undesired noise signals mix with the desired signals caused by the string vibration and degrade the quality of the resulting composite signal in that it is not pure string signal.
To combat noise, workers in the prior art have developed various pickup designs which are adapted to minimize noise pickup. The original noise cancelling pickup design in the prior art was made by Lover and patented as U.S. Pat. No. 2,896,491. This design was a side-by-side two-coil magnetic pickup. A first coil is designed to pick up mostly string signal but it also picks up some noise. A second coil is designed to pick up more noise than string signal. The first coil has a magnet which has a north polarity and the second coil has a magnet which has a south polarity. The coils are connected so that the signal from one coil is 180 degrees out of phase with the signal from the first coil when the two signals are added. In Lover, the string signals are additive because the opposite polarities create opposite phase string signals, but the out of phase connection of the coils reverses the effect of the opposite polarity thereby causing the string signals to add. This causes larger string signal output. However, hum signal in the coils is not caused by the magnetic field of the coil magnets so hum signal has the same polarity in both coils. Because the two coils are coupled so as to be 180 degrees out of phase, the hum signals cancel.
The disadvantage of the side-by-side arrangement of Lover is that the string signal is picked up by the two coils based upon vibrations at two different points in the string. Because high frequency harmonics have very short wavelengths, the string signal from these high frequency harmonics is not the same in both magnets. As a result, the low frequency harmonics whose wavelengths are long enough that the two different points problem has no effect will have their signal added whereas high frequency harmonics will not. This reduces the fidelity of the reproduction of the actual string vibrations and causes the pickup to have a muted sound which is lacking in detail.
Another example of a prior art noise cancelling pickup is U.S. Pat. No. 3,657,461 to Freeman. This was also a two-coil, noise-cancelling pickup with the coils stacked vertically  and wrapped around bar magnets with a divider in between the coils.
More recently, U.S. Pat. No. 4,442,749 issued to DiMarzio et al. This patent taught a two-coil, noise-cancelling pickup with the coils stacked vertically and wrapped around a plurality of rod-like permanent magnets. The two coils were wrapped around a pair of superposed coaxial bobbins and oriented such that the axis of the coils was perpendicular to the plane of the strings. An integral plate of magnetic material is provided comprising a base disposed between the two bobbins perpendicular to the coil axis and the two side walls extending upward and perpendicular to the base to at least immediatly below the top face of the upper bobbin to act as a shield of the top coil. In other words, a shield of magnetic material having a plate parallel to the plane of the strings and separating the two bobbins was incorporated, and the plate had two vertical sidewalls orthogonal to the plane of the strings and covering the sidewalls of the upper coil to shield it from noise flux. The rod-like permanent magnets contact the base of the integral plate with all rod magnets having like polarity at the tops thereof. The upper and lower coils were wound in opposite directions so the noise signal generated by the lower coil was 180 degrees out of phase with the string signal. The idea was to use the shield to prevent noise electromagnetic fluctuations from reaching the top coil windings to generate currents therein. The signal from the bottom coil was not shielded and picked up noise signal which cancelled part of the noise signal from the top coil.
U.S. Pat. No. 4,524,667 to Duncan teaches a two-coil, noise-cancelling pickup where the two coils are vertically stacked around the permanent magnets which extend through the centers of the two windings. See FIG. 5 for the configuration. A switching circuit allows the two coils to be connected in either a single or dual coil configuration.
U.S. Pat. No. 5,668,520 to Kinman teaches a two-coil, noise-cancelling pickup with the axes of the coils coincident and using six magnetized rod permanent magnets extending as pole pieces up through the axis of the first coil coils and six non-magnetized pole pieces extending through the axis of the second coil, all pole pieces having long axes which are orthogonal to the plane of the strings. Both multiple rod magnet pole pieces and blade magnet pole pieces are disclosed. Two U-shaped shields which are back to back with sidewalls that shield the sides of the first and second coils serve in both embodiments to shield the first and second coils from each other both magnetically and inductively.
U.S. Pat. No. 5,834,999 to Kinman is a continuation-in-part of U.S. Pat. No. 5,668,520 and teaches a two-coil, noise-cancelling pickup with substantially the same configuration as the parent patent but the shield does not extend as far in the horitonal direction toward the end of the racetrack shaped (two long straightaways coupled by tight turns at the ends) coils.
U.S. Pat. No. 6,103,966 to Kinman is a continuation-in-part of U.S. Pat. No. 5,834,999 and teaches a two-coil, noise-cancelling pickup with substantially the same configuration as the parent patent but teaching a variety of different pole piece configurations.
U.S. Pat. No. 6,291,759 to Turner teaches a two-coil, noise-cancelling pickup comprising an upper bobbin, a ferromagnetic steel plate and a lower bobbin, stacked on top of each other, oriented longitudinally and laterally substantially the same, and held together by ferromagnetic screws. An upper coil is wound around a middle section of the upper bobbin, and a lower coil is wounded in an opposite manner around a middle section of the lower bobbin, whereby the upper and lower coils are connected in series. The upper and lower bobbins, and steel plate each include a plurality of coaxial apertures to receive corresponding permanent magnetic pole pieces that extend from the upper bobbin to the lower bobbin. The key difference over the prior art appears to be that the upper and lower bobbins include additional apertures to receive ferromagnetic cylinders to selectively change the tonal characteristics of the guitar. The pickups may include a pair of ferromagnetic plates (64 in FIG. 11) attached to the longitudinal sides of the lower bobbin that extend upwards to about the middle of the upper coil. These ferromagnetic plates are electrically insulated from the pole pieces. The purpose of the steel plates 64 is to concentrate the electromagnetic fields generated by the permanent-magnet pole pieces 62 around the coils 58 and 60 of the pickup 50. The concentrated electromagnetic fields around the coils 58 and 60 increase the coupling between the electromagnetic sensing of the string vibration and the voltage produced at the pickup electrical connection. This results in a more efficient generation of voltage at the coil ends or electrical connections of the pickup 50.
U.S. patent application Ser. No. 09/909,473 filed 4 Jul. 2002, published as U.S. 2002/0083819, inventor Kinman, teaches a low eddy current core in a noise cancelling pickup coil.
Other U.S. patents which teach related subject matter are: U.S. Pat. No. 3,236,930 to Fender teaching a single coil pickup with shaped sidewalls; U.S. Pat. No. 3,915,048 to Stich teaching a switching system for noise cancelling pickups; U.S. Pat. No. 4,026,178 to Fuller teaching a single coil pickup with shaped sidewalls; U.S. Pat. No. 4,133,243 to DiMarzio teaching a pickup with adjustable pole pieces; U.S. Pat. No. 4,220,069 to Fender teaching a single coil pickup with sidewalls; U.S. Pat. No. 4,283,982 to Armstrong teaching variations in magnet and coil placement in side-by-side noise cancelling design; U.S. Pat. No. 4,809,578 to Lace teaching a single coil pickup with sidewalls; U.S. Pat. No. 5,464,948 to Lace teaching a single coil pickup with sidewalls; U.S. Pat. No. 5,811,710 to Blucher teaching tapered/stepped sidewalls in a stack-type noise cancelling design; U.S. Pat. No. 5,908,998 to Blucher teaching teaches extra metal slugs to increase the inductance of the lower coil; and U.S. Pat. No. 6,111,185 to Lace teaching horizontal coils with side walls.
In the prior art of which the applicant is aware, both the upper and lower coils of the pickup are typically of the same physical size. In the most recent prior art, different approaches such as using different wire guages and different numbers of turns on the upper and lower coils in an attempt to reduce the size of the pickup without losing the hum cancellation tendancy of having a two coil pickup. Typically, the upper coil is wound with a high number of turns of a lighter guage wire and the lower coil is wound with a lower number of turns of a heavier guage wire. Hum cancellation is usually accomplished by some combination of shielding the upper coil with ferrous plate and/or increasing the inductance of the lower coil. Increasing the inductance of the lower coil is typically done by iron loading (adding extra iron beside the pole pieces in the central cavity of the lower coil). The intent of these different approaches is to decrease the amount of hum signal in the upper coil compared to the string signal and to increase the amount of hum signal in the lower coil such that this signal can be used to cancel hum signal in the upper coil. These prior art approaches have several shortcomings.
First, the upper and lower coils are always the same size. This is because the other techniques such as shielding and inductance maximization cannot alone create enough hum cancellation without having the upper and lower coils the same size. In other words, it is necessary to have the lower coil the same size as the upper coil in order to get enough hum signal in the lower coil to cancel the hum signal still left in the upper coil after shielding.
Second, it is highly desirable to emulate with a two-coil pickup the sound of a single coil pickup because musicians prefer the sound of the single coil pickup but hate hum. However, because both coils in the two coil pickups are the same size, and the lower coil is typically filled with iron load, the magnetic structure is necessarily significantly different from the single coil pickup. Two coil pickups have shorter pole length and a shorter coil profile, for example than single coil pickups. The different magnetic and mechanical structures produce different output and attach characteristics. However, the desire is to have a two coil pickup with the same sound as a single coil pickup but with less hum. Preferably, a two-coil stacked pickup which improves over the prior art would be small enough to retrofit into the pickup cavity of prior art stringed instruments.
Some prior art designs have tried to get closer to the sound of a single coil pickup by using high magnetic strength rare earth magnets in two coil pickups. But this high magnetic field results in excessive string damping (the strings are metal and are subjected to physical forces by the high magnetic field which alters their vibration pattern) and production of “false harmonics” both of which phenomena alter the sound of the guitar.
Third, because the upper string sensing coil is the same size as the lower coil, the upper sensing coil will always have a different geometry and wire guage from the traditional single coil pickup. This is because if the geometry were the same in the coils of a two coil pickup as in a single coil pickup, the two coil pickup would be much too large to fit in the space available for the pickup in traditional instruments without modifying the instrument. If the same wire guage were to be used in a two coil pickup as is used in traditional single coil pickups, the larger wire size would require that the two coil pickup coils would have fewer turns than the single coil pickup coil so that the two coil pickup could be made small enough to fit into the available space. The fewer number of turns means a smaller signal would be output from the pickup thereby requiring more amplification. A lower number of turns also gives a higher resonant frequency in addition to lower output. Both these characteristics alter the sound output from the pickup. Amplification also amplifies any residual hum signal in the pickup output so the hum becomes louder and more distracting. The shorter coil geometry forced on the two coil pickups by the space limitations means that the geometry of the single coil pickup is not faithfully reproduced which results in loss of faithful reproduction of the single coil pickup sound.
The prior art designs also fail to adjust for normal production variations in the manufacture of the pickups. The manufacturer will therefore have variations in hum signal from one pickup to the next, or, if strict quality control standards are imposed, a higher than normal reject rate.